Throughout this application various publications are referenced by the names of the authors and the year of the publication within parentheses. Full citations for these publications may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
Inductive interactions that define the fate of cells within the neural tube establish the initial pattern of the embryonic vertebrate nervous system. In the spinal cord, the identity of cell types is controlled, in part, by signals from two midline cell groups, the notochord and floor plate which induce neural plate cells to differentiate into floor plate, motor neurons and other ventral neuronal types (van Straaten et al. 1988; Placzek et al. 1990, 1993; Yamada et al. 1991; Hatta et al. 1991). The induction of floor plate cells appears to require a contact-mediated signal (Placzek et al. 1990a, 1993) whereas motor neurons can be induced by diffusible factors (Yamada et al., 1993). Thus, the fate of different ventral cell types may be controlled by distinct signals that derive from the ventral midline of the neural tube.
The specification of dorsal cell fates appears not to require ventral midline signals since the neural tube still gives rise to dorsal cell types such as sensory relay neurons and neural crest cells after elimination of the notochord and floor plate (Yamada et al. 1991; Placzek et al. 1991; Ericson et al. 1992). Moreover, dorsal cell types are found at more ventral positions in such embryos (Yamada et al. 1991; Placzek et al. 1991) suggesting that many or all cells in neural tube have acquired dorsal characteristics. The acquisition of a dorsal fate could represent a default pathway in the differentiation of neural plate cells or a response to inductive factors that are distinct from the ventralizing signals that derive from the notochord and floor plate.
To identify signals that might regulate cell differentiation within the neural tube, genes encoding secreted factors that are expressed in a restricted manner along the dorsoventral axis of the neural tube have been searched. In this application, the transforming growth factor .beta. (TGF .beta.) family have been focused since some of its members have been implicated in the control of cell differentiation and patterning in non-neural tissues. In frog embryos, for example, the differentiation and patterning of mesodermal cell types appears to be controlled, in part, by the action of activin-like molecules (Ruiz i Altaba and Melton, 1989; Green and Smith, 1990; Thomsen et al. 1990; Green et al. 1992). In addition, the dorsoventral patterning of cell types in Drosophila embryos is regulated by the decapentaplegic (dpp) gene (Ferguson and Anderson, 1992a,b). The dpp protein is closely related to a subgroup of vertebrate TGF .beta.-like molecules, the bone morphogenetic proteins (BMPs) (Wozney et al. 1988), several members of which are expressed in restricted regions of the developing embryos (Jones et al. 1991). In this application, the cloning and functional characterization of the dorsalin-1 (dsl-1) gene, which encodes a novel BMP-like member of the TGF-.beta. superfamily are described. Dsl-1 is expressed selectively by cells in the dorsal region of the neural tube and its expression in ventral regions appears to be inhibited by signals from the notochord. Dsl-1 promotes the differentiation or migration of neural crest cells and can prevent the differentiation of motor neurons in neural plate explants. The combined actions of dsl-1 and ventralizing factors from the notochord and floor plate may regulate the identity of neural cell types and their position along the dorsoventral axis of the neural tube.